My wife is angmoh and gets approached a lot by locals (especially malay) so maybe some local guys are more shy or don't like angmoh gers but there are certainly ones that are confident and many guys actually also like angmoh gers. Even if the pickup lines are stupid "you angmoh ar?", she doesn't mind the attention!
I think the reasons are complex. But part of it is we all kenna brainwash by western media and associate angmohs with prestige. tv and movies oso emphasize western faces. Part of it oso angmohs maybe less picky in choosing which local ger to have a fling wif.